Step 2 - Automated processing procedure

Determining positions using light‐level geolocation can be achieved using threshold methods and curve‐fitting methods (Lisovski et al. 2020). Using the light-level and time information contained in the raw logger data (a), we applied an automated processing procedure to determine, filter, and edit twilight events and calculate geographic positions using the threshold method (Figure 1). The procedure is described in detail in Bråthen et al. (2021). It further removes unrealistic positions using filters on equinox periods, speed, distribution, angle, distance, variation in timing of twilights and mid-night sun periods and smooths positions. The one aspect not automated is the calibration of sun elevation angles, crucial to estimate the latitudinal aspect of positions. It is based on a subjective, but high repeatability assessment for each annual track for each bird.

Figure 1: Conceptual illustration of information used to estimate locations from light level data using the threshold method.

The automated processing procedure yields a filtered location dataset (b) for each of the eleven SEATRACK species containing up to two locations per bird and day (one location at noon and one at midnight), with a total of 3 945 645 unique processed locations from 3 933 individuals for the eleven species tracked (Table 1).

Table 1: Summary of tracking data collected as part of SEATRACK inside and outside the Norwegian Ocean Management areas (Figure 3). Number of colonies where individuals have been tracked, as well as total number of individuals, annual tracks (often incomplete) and positions obtained over the project period.
Species N colonies N inds N tracks N pos
Black-legged kittiwake 22 981 2 267 1 111 256
Brünnich’s guillemot 16 519 1 213 521 348
Herring gull 9 57 128 66 517
Northern fulmar 16 346 892 401 803
Atlantic puffin 17 531 1 071 461 379
Common guillemot 13 523 1 252 616 103
European shag 6 248 539 215 696
Glaucous gull 4 117 233 79 584
Little auk 5 297 392 148 558
Common eider 7 276 571 280 017
Lesser black-backed gull 5 38 82 43 384
Total 55 3 933 8 640 3 945 645